I. Field of the Invention
This invention relates generally to on-demand implantable cardiac rhythm management devices, such as implantable pacemakers and automatic implantable cardiac defibrillators, and more particularly to a novel design of sensing circuitry for detecting the occurrence of cardiac depolarization, either P-waves or R-waves, in the presence of muscle noise and other ECG artifacts.
II. Discussion of the Prior Art
In implantable cardiac pacemakers and/or cardioverters /defibrillators employing an R-wave detector, or both an R-wave detector and a P-wave detector, it is imperative that R-waves and/or P-waves be reliably detected even in the presence of noise which may be picked up on the cardiac leads and delivered to the implanted device. Noise sources typically include 50 or 60 Hz power line noise, muscle noise, motion artifacts, baseline wander and T-waves. A cardiac event is sensed when the amplified and filtered input signal, such as a P-wave or an R-wave, exceeds an established threshold value which is generally programmed into the device at the time of implantation.
In accordance with the prior art, the sensing threshold is static in time. It is adjusted by the physician to a level that is considered to be the best compromise for sensing the R-waves or P-waves seen at the time of adjustment and for noise avoidance. If the gain (sensitivity) of the sense amplifier is set too high, noise may be able to trigger the comparator and give a false indication of a cardiac event. Alternatively, if the gain or sensitivity is set too low, a legitimate cardiac event may not be detected by the comparator.
It is known in the art to provide upper and lower target levels where the lower level is approximately one-half of the amplitude of the upper level. The automatic sensing system attempts to maintain the peak of the R-wave between these two target levels and ideally bumping the upper level. In this regard, reference is made to a paper entitled "Clinical Evaluation of an Automatic Sensitivity Adjustment Feature in a Dual Chamber Pacemaker" by Wilson et al., Pace, vol. 13, pp. 1220-1223, October 1990. In this paper, the threshold is described as being increased after a predetermined number of beats are found to exceed the upper target, and decreased when a second predetermined number of beats exceeds the lower but not the upper target. This arrangement has a significant drawback in that it gives a very slow response to changes in R-wave amplitude.
The Gobeli et al. U.S. Pat. No. 3,927,677 describes a system where the comparator trip point is varied to sense at some level above the average value of the input signal. This offers the advantage of providing noise immunity, particularly to continuous noise such as 50-60 Hz pickup from household appliances and the like. The Keimel U.S. Pat. No. 5,117,824 describes the concept of using a proportion, e.g., 75%, of the peak R-wave amplitude as the initial value of the comparator trip point. The initial value is made to decay to some minimum value over a time period of three seconds or less.
It is also known in the art to provide automatic gain control (AGC) to adjust the gain of a variable gain sense amplifier to effectively vary the sensitivity of the sense amplifier so that the cardiac depolarization signal stays within the dynamic range of the sense amplifier. In this regard, reference is made to the Hamilton et al. U.S. Pat. No. 4,708,144 and the Baker, Jr. et al. U.S. Pat. No. 4,903,699 and the Keimel et al. U.S. Pat. No. 5,117,824.